1. Field of the Invention
The present invention relates to an LED display apparatus in which switching elements are parallel-connected to multiple serially-connected LEDs, respectively, and turned on/off to display figures and the like.
2. Description of the Related Art
LED display apparatuses using multiple LEDs to display figures and the like are known. The normally-on LED display apparatus must satisfy general requirements such as small size, low price, and high quality display and other performances, particularly low power consumption and operation with a low voltage power source.
Such LED display apparatuses are known from the Japanese Patent Publication Nos. 9 (1997)-81211 (Patent Document 1), 5 (1993)-129665 (Patent Document 2), 5 (1993)-131681 (Patent Document 3), 8 (1996)-194448 (Patent Document 4), and 2001-109433 (Patent Document 5). Patent Document 1 discloses a display circuit having a constant current power source, multiple serially-connected light emitting elements serially-connected to the constant current power source, and bypass circuits provided one each to the light emitting elements and parallel-connected to the corresponding one of them wherein the bypass circuit shorts between the ends of the corresponding light emitting element at sufficiently smaller resistance than that of the corresponding light emitting element in response to supplied control signals. Patent Document 2 discloses an LED drive circuit comprising a set of light emitting elements serially-connected in the forward direction, a constant current power source supplying the set of light emitting elements with a constant current, a set of switching elements each parallel-connected to one of the set of light emitting elements to turn on/off the corresponding light emitting element based on external control signals. Patent Document 3 discloses an LED array apparatus comprising an LED array consisting of a number of LEDs juxtaposed and a circuit controlling the conduction of the individual LEDs wherein the LED array apparatus is provided with an LED array consisting of serially-connected LEDs, switching elements each parallel-connected to one of the LEDs, and a constant current unit powering the LED array. Patent Document 4 discloses a display apparatus comprising a light source constituted by a serial element consisting of multiple serially-connected LEDs wherein a constant current element is serially-connected to the serial element. Patent Document 5 discloses a dot matrix display apparatus in which multiple scanning electrodes and multiple signal electrodes intersect in a matrix form and a display element is driven by the voltage between the scanning electrode and the signal electrode at each intersection of the matrix wherein a rectifying element is electrically connected in a specific electrode direction between a scanning electrode and a reference voltage terminal supplying a given reference potential and the charge on the scanning electrode is discharged to the reference voltage terminal via the rectifying element.
However, the above described prior art LED display apparatuses have the following problems.
First, a necessary number of LEDs are serially-connected in Patent Documents 1, 2 and 3. Therefore, in Patent Documents 1, 2 and 3, a higher drive voltage is required in correspondence with the number of LEDs to be used, leading to a larger power source and increased cost. If a higher drive voltage is not available, the number of LEDs to be used is limited or their luminance may be lowered. When the LEDs are parallel-connected, the luminance may not be uniform and the power consumption is increased.
A necessary number of LEDs are used in a combination of serial-connection and parallel-connection, in other words they are connected in a matrix array in Patent Documents 4 and 5. When a matrix connection is used, the drawbacks with the serial-connection and parallel-connection can be eliminated to a certain extent. However, the parallel circuits (row) or serial circuits (column) are controlled as a group. Therefore, the individual LEDs are not controlled, disadvantageously limiting display applications.